The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving ways, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Generally, an AMOLED pixel driving circuit comprises two thin film transistors (TFT) and one capacitor, i.e. a 2T1C pixel driving circuit. The thin film transistor employed for controlling the writing of the data signal (Data) is a Switching TFT, and the thin film transistor employed for controlling the current flowing through the OLED is a Driving TFT. Therefore, the importance of the threshold voltage (Vth) of the driving TFT is obviously significant. Both the positive and negative drifts of the threshold voltage will make different current flowing through the OLED under the same data signal and the OLED will have different brightness.
At present, both the thin film transistors manufactured by Low Temperature Poly-silicon (LTPS) and oxide semiconductor will have the phenomenon of threshold voltage drift during usage. For example, the factors of the light irradiation, source-drain voltage stress and etc. can cause the threshold voltage drift and result in that the current flowing through the OLED is not consistent with the desired current. The panel brightness cannot satisfy the requirements, therefore. In the general 2T1C pixel driving circuit, the threshold voltage drift of the Driving TFT cannot be improved by adjustment. Thus, new thin film transistors or new signals are required to weaken or even eliminate the influence caused by the threshold voltage drift.
Please refer to FIG. 1, which shows an AMOLED pixel driving circuit utilizing a 5T1C structure according to prior art, comprising: a first thin film transistor T1, a second thin film transistor T2, a third thin film transistor T3, a fourth thin film transistor T4, a fifth thin film transistor T5, a capacitor Cst and an organic light emitting diode OLED. A gate of the first thin film transistor T1 is electrically coupled to one end of the capacitor Cst and a drain of the second thin film transistor T2, and a source is electrically coupled to a drain of the third thin film transistor T3, and a drain is electrically coupled to a drain of the fourth thin film transistor T4 and a source of the fifth thin film transistor T5; a gate of the second thin film transistor T2 is electrically coupled to a first scan control signal N1, and a source is electrically coupled to the drain of the third thin film transistor T3 and a drain is electrically coupled to the gate of the first thin film transistor T1 and the one end of the capacitor Cst; a gate of the third thin film transistor T3 is electrically coupled to a light emitting control signal EM, and a source is electrically coupled to a power supply positive voltage SR1/OVDD and a drain is electrically coupled to the source of the second thin film transistor T2 and the source of the first thin film transistor T1; a gate of the fourth thin film transistor T4 is electrically coupled to the first scan control signal N1, and a source is electrically coupled to a data signal DIN/VData, and a drain is electrically coupled to the drain of the first thin film transistor T1 and a source of the fifth thin film transistor T5; a gate of the fifth thin film transistor T5 is electrically coupled to the light emitting control signal EM, and a source is electrically coupled to the drain of the first thin film transistor T1 and the drain of the fourth thin film transistor T4, and a drain is electrically coupled to an anode of the organic light emitting diode OLED; the one end of the capacitor Cst is electrically coupled to the gate of the first thin film transistor T1 and the drain of the second thin film transistor T2, and the other end is electrically coupled to a second scan control signal N2; the anode of the organic light emitting diode OLED is electrically coupled to the drain of the fifth thin film transistor T5, and a cathode is electrically coupled to a power supply negative voltage SR2/OVSS. In the present 5T1C AMOLED pixel driving circuit, the other end of the capacitor Cst needs the second scan control signal N2 to be inputted solo for controlling. Consequently, the input signal is complicated. The manufacture cost of the panel is higher and the stability of the circuit is worse.